Many plants contain chemical constituents which have medicinal or pharmaceutical activity and are commercially grown or gathered for that reason. Commercially important examples include St. John's wort (Hypericin perforatum), and echinacea (Echinacea purpurea, E. augustifolia, or E. pallida).
St. John'swort, Hypericum perforatum (L.) is a perennial herbaceous plant widespread in Europe and the Americas. The plant contains hypericin and its analog pseudohypericin and both have implications in the analgesic, antimicrobial, anti-inflammatory, antioxidant and antidepressant activities of the plant. Air-drying of the herb however reduces the level of hypericin by up to 80%, most likely as a result of oxidation (O. S. Araya and J. H. Ford (1981). An investigation of the type of photosensitization caused by the ingestion of St. John's wort Hypericum perforatum by calves. Journal of Comparative Pathology 135-141).
Echinacea plant materials are believed to have antiviral, antibacterial, and antifungal properties due to their non-specific enhancement of mammalian immune systems (Wagner et al 1988, Roesler et al, 1991; Steinmuller et al., 1993). Echinacea plant materials are also reported to have anti-inflammatory properties (Tubaro et al. 1987; Bauer and Wagner 1991, Muller-Jakic, 1994). Commercial plant preparations are produced from the aerial parts of E. purpurea and the underground parts of E. purpurea, E. angustifolia, and E. pallida. Although some uncertainly still exists as to the exact components of echinacea responsible for its medicinal activity, the group of compounds called alkamides are the most likely candidates. Alkamides are isobutylamides of highly unsaturated carboxylic acids with olefinic and/or acetylenic bonds (Greger, 1984). Using High Pressure Liquid Chromatography (HPLC), researchers have isolated and identified individual alkamides in echinacea. Bauer and Remiger (1989) identified 11 alkamides from the roots of E. purpurea. Different preparations of echinacea are currently in use around the world. Both fresh and dried forms of echinacea plant parts are used to make juice, powders, tablets, tinctures and capsules.
Many medicinal plant materials are unstable as they are harvested and must be dehydrated to render them sufficiently stable to be marketed or further processed. Dehydration may take the form of simple solar drying in the field but this practice renders the products susceptible to contamination by insects, microorganisms and general filth as well as the vagaries of weather. Commercial hot-air dehydrators powered by fossil fuels or electricity provide a more controlled and reliable drying option. None the less, a substantial portion of the active chemical constituents may be lost during the drying process due to the combination of high temperatures and atmospheric oxygen in the drying environment. These factors promote chemical oxidation of the active constituents, rendering them inactive, as indicated above for St. John's wort. The alkamides of echinacea, being unsaturated, that is containing double carbon bonds within their molecular structure, are likewise susceptible to destruction by interaction with oxygen. Elevated temperatures also promote oxidative reactions.
Durance and Liu, 1996. "Production of Potato Chips" U.S. Pat. No. 5,676,989 Teaches a process for simulating frying of snack foods such as potato chips by the application of microwave power under variable levels of vacuum in order to create the texture and flavor of frying without the use of vegetable oil.
Durance et al., 1998 "Process for Drying Herbs" (U.S. patent application Ser. No. 09/081,212) teaches a process for dehydrating culinary herbs in which retention of volatile flavor compounds is desirable, wherein vacuum microwave drying is employed to reduce the drying temperature and increase drying rate. In that process rapid, low temperature dehydration results in improved retention of volatile, low molecular weight flavor compounds because low temperature reduces evaporation rate of the flavor compounds and low temperature and short drying times do not allow time for the volatile compounds to diffuse out of the herb tissue into the drying chamber from hence they are lost.